Detail

The Scrollbar widget is an abstract base class for
Graphics.UI.Gtk.Scrolling.HScrollbar and
Graphics.UI.Gtk.Scrolling.VScrollbar. It is not very useful in itself.

The position of the thumb in a scrollbar is controlled by the scroll
adjustments. See Graphics.UI.Gtk.Misc.Adjustment for the fields in an
adjustment - for
Scrollbar, the "value" field represents the position of the scrollbar,
which must be between the "lower" field and "upper - page_size." The
"page_size" field represents the size of the visible scrollable area. The
"step_increment" and "page_increment" fields are used when the user asks
to step down (using the small stepper arrows) or page down (using for
example the PageDown key).